1. Field of the Invention
The present invention relates to a sheet that is coated with silicone compounds in order to heighten the effect of preventing adhesion of stains and a process for preparing the sheet. More particularly, the invention relates to a sheet highly effective in preventing adhesion of stains which has a multi-layer construction coated with silicone compounds different in surface tension and a process for preparing the sheet.
2. Description of the Related Art
In recent years, there are a variety of sheets formed of fabric, non-woven fabric, or resin sheets that have undergone a water-repelling treatment. Examples of such sheets include raincoats, bibs for babies and infants, aprons, lunch mats, tablecloths, boots, underwear, and back sheets of sanitary goods such as diapers and sanitary napkins. This treatment is carried out to prevent adhesion of stains, to allow stains to wash out easily even though the sheets are stained, or to prevent water or stains from oozing through the sheets.
For the water-repelling treatment, sheets are coated, for example, with rubber, resins, or silicone oil. For example, Japanese Patent Laid-Open No. 194183/1985 discloses cloth where a fiber construction treated with a silicone type water repellent is coated with film formed of silicone rubber. However, this cloth can not prevent adhesion (sticking) of stains effectively even though it can repel water.
In order to prevent adhesion (sticking) of stains, sheets need still higher water repellency. For example, blood is a liquid that contains water as solvent. The surface tension of the water is about 73.0 mN/m in its standard condition and on the other hand, the surface tension of the blood is about 54.0 mN/m. Accordingly, for preventing adhesion of stains such as blood, the sheets require still higher water repellency. In addition, for blood or a protein-containing liquid to be scattered at meals, it is irrational to evaluate its adhesion only by surface tension. For example, albumin and globulin in blood and various proteins contained in a liquid to be scattered at meals have a three-dimensional structure and in addition, have various groups such as amino groups and carboxyl groups. Consequently, protein-containing stains exhibit very high adhesion as compared with water molecules having a strong tendency to cohere to themselves, caused by hydrogen bonds. Therefore, the development of sheets that have still higher water repellency and are highly effective of preventing adhesion of stains has been in demand.
In addition, conventional water repellents having low resistance to friction have a disadvantage of deteriorating in water repellency due to flaking off or producing cracks on the surface by the use in a short period of time. For improvement in anchoring effect to base materials, modification of the molecular structures of the water repellents themselves or addition of an anchoring agent (such as hot melt adhesive, urethane curing agent or the like) to the water repellents is carried out. However, practice of these methods results in deterioration in water repellency of the repellents.
The present invention is to solve the problems described above and has an object to provide a sheet having high water repellency that can prevents adhesion of protein-containing stains.
Another object of the invention is to provide a sheet that can maintain high water repellency and high effectiveness in preventing adhesion of stains in spite of friction working on its surface.
According to a first aspect of the invention, there is provided a sheet highly effective in preventing adhesion of stains, which comprises a base sheet, a first silicone compound layer formed on the surface of the base sheet, and a second silicone compound layer further formed on the surface of the first silicone compound layer, the second silicone compound layer having a lower surface tension than the first silicone compound layer. A process for preparing this sheet comprises the steps of forming a primary coat of the first silicone compound (i.e., the first silicone compound layer) on the surface of the base sheet; curing the primary coat; and forming a surface coat of the second silicone compound (i.e., the second silicone compound layer) having a lower surface tension than the first silicone compound on the surface of the primary coat.
According to a second aspect of the invention, there is provided a sheet highly effective in preventing adhesion of stains, which comprises a base sheet and a layer formed on the surface of the base sheet, wherein the layer comprises a mixture of a first silicone compound and a second silicone compound having a lower surface tension than the first silicone compound, and the second silicone compound appears on the surface of the layer more than the first silicone compound. A process for preparing this sheet comprises the steps of: forming a coat of a mixture of the first silicone compound and the second silicone compound having a lower surface tension than the first silicone compound on the base sheet; and crosslinking the silicone compounds in a condition where the second silicone compound is transferred to the surface side of the coat and first silicone compound is transferred to the base sheet side of the coat. Alternatively, the sheet may be prepared by a process comprising the steps of: forming a coat of a mixture of the first silicone compound and the second silicone compound having a lower surface tension than the first silicone compound on the base sheet; and crosslinking the silicone compounds so that the second silicone compound appears on the surface of the coat more than the first silicone compound by bleeding.
These sheets of the invention are coated with silicone on the surfaces and therefore, exhibit high water repellency. Moreover, they can prevent to a great extent the adhesion of protein-containing stains. In these sheets, still moreover, friction of the surfaces coated with the silicones does not cause deterioration in these effects.
The term xe2x80x9csurface tensionxe2x80x9d as used for the silicone compounds in the invention means a surface tension of silicone compound in a liquid state. However, even after the silicone compounds are fixed (or solidified), it is preferable that the surface tension of the first silicone compound is higher than the surface tension of the second silicone compound.
In accordance with the invention, the presence of the first silicone compound having a certain surface tension on the base sheet side and the presence of the second silicone compound having a lower surface tension than the first silicone compound on the surface side result in heightening the effect of preventing the adhesion of stains. The sheets of the invention that are highly effective in preventing the adhesion of stains can be prepared by any one of the following three methods.
(1) A method where the first silicone compound is applied to the surface of a base sheet and then crosslinked to form a primary coat (layer) and after fixing the primary coat, the second silicone compound having a lower surface tension than the first silicone compound is applied to the surface of the primary coat to form a surface coat (layer).
(2) A method where a mixture of the first silicone compound and the second silicone compound having a lower surface tension than the first silicone compound is applied to the surface of a base sheet to form a coat (layer) and the silicone compounds are crosslinked in a condition where the second silicone compound is transferred to the surface side of the coat and the first silicone compound is transferred to the base sheet side of the coat.
(3) A method where a mixture of the first silicone compound and the second silicone compound having a lower surface tension than the first silicone compound is applied to the surface of a base sheet to form a coat (layer) and after crosslinking, the major portion of the second silicone compound is allowed to come to the surface of the coat by bleeding.
In method (1), the first silicone compound is applied and fixed to the base sheet and subsequently, the second silicone compound is applied to form a double-layer construction. Accordingly, either of a reactive polymer and a non-reactive compound can be used as the first and the second silicone compounds.
In methods (2) and (3), it is preferred that a reactive polymer is used as the first silicone compound and a non-reactive compound is used as the second silicone compound. In method (2), when the mixture of the first silicone compound and the second silicone compound is allowed to stand for a while in a condition where the mixture has been coated on the base sheet, the first silicone compound is transferred to the base sheet side of the coat and the second silicone compound is bled to the surface side of the coat. In this condition, crosslinking is carried out to fix the silicones on the base sheet.
In method (3), immediately after the mixture of the first and the second silicone compounds is applied to the base sheet, a crosslinking treatment such as heating is carried out. The second silicone compound undergoes bleeding to the surface side of the coat by this treatment. This phenomenon seems to occur because the surface tension of the second silicone compound formed of the non-reactive compound is lower than that of the first silicone compound. In the sheet of the invention prepared by method (3), the amount of the second silicone compound gradually increases toward the layer""s surface away from the base sheet.
The first silicone compound is illustrated below.
In order to heighten the effect of preventing the adhesion of stains in combination with the second silicone compound, the first silicone compound preferably has a surface tension of 30.0 mN/m or less. In the invention, the surface tensions were determined at standard conditions (atmospheric temperature of 20xc2x0 C.).
The first silicone compound is preferably reactive silicone oil containing siloxane structure as a fundamental skeleton. For example, the compound is preferably at least one compound selected from the group consisting of methylhydrogensilicone oil, polydimethylsiloxanediol, epoxy-modified silicone oil, carboxyl-modified silicone oil, methacryl-modified silicone oil, alcohol-modified silicone oil, mercapto-modified silicone oil, vinyl-modified silicone oil, and amino-modified silicone oil. These compounds can be fixed on the base sheet by subjecting them to a process of drying, heating or the like, after coating.
In method (1), the silicone compound can also be used as the first compound. It is preferred that the silicone compound is a compound that is preferably at least one compound selected from the group consisting of dimethylsilicone oil, methylphenylsilicone oil, alkyl-based silicone oil, aralkyl-based silicone oil, polyether-modified silicone oil, fluroalkyl-modified silicone oil, and aliphatic ester-modified silicone oil.
In method (1), in addition, examples of the first silicone compound may include polysiloxane; silicone varnish such as silicone alkyd varnish, silicone epoxy varnish, silicone polyester varnish, silicone acryl varnish, silicone phenol varnish, silicone urethane varnish, and silicone melamine varnish; and silicone rubber such as dimethylsilicone rubber, methylvinylsilicone rubber, methylphenylvinyl-silicone rubber, and methylfluoroalkylsilicone rubber.
The second silicone compound is illustrated below.
The surface tension of the second silicone compound is lower than that of the first silicone compound. The surface tension of the second silicone compound is preferably at least 5.0 mN/m lower than that of the first silicone compound. When the first silicone compound having a surface tension of 30.0 mN/m or less is used, the surface tension of the second silicone compound is preferably 21.0 mN/m or less.
In method (1), the second silicon compound is preferably at least one compound selected from the group consisting of dimethylsilicone oil, methylphenylsilicone oil, methylhydrogensilicone oil, polydimethylsiloxanediol, alkyl-based silicone oil, aralkyl-based silicone oil, polyether-modified silicone oil, fluoroalkyl-modified silicone oil, aliphatic ester-modified silicone oil, amino-modified silicone oil, epoxy-modified silicone oil, carboxyl-modified silicone oil, methacryl-modified silicone oil, alcohol-modified silicone oil, mercapto-modified silicone oil, and vinyl-modified silicone oil. Preferably, the second compound is a silicone oil.
In methods (2) and (3), the second compound is preferably a silicone compound that is, for example, at least one compound selected from the group consisting of polysiloxane, silicone alkyd varnish, silicone acryl varnish, silicone phenol varnish, silicone urethane varnish, silicone melamine varnish, and dimethylsilicone rubber.
In the invention, any material can be used as the base sheet to be coated with the silicone compounds. For example, the base sheet is formed of at least one compound selected from the group consisting of: cellulose compounds such as cupra, rayon, and acetate; polyamide compounds such as nylon 6, nylon 66, and aromatic nylon; polyvinyl alcohol compounds such as Vinylon; polyvinylidene chloride compounds such as vinylidene; polyvinyl chloride compounds such as polyvinyl chloride; polyester compounds such as polyethylene terephthalate, and polyacrylate; polyacrylonitrile compounds such as acryl; polyethylene compounds such as low-density polyethylene, high-density polyethylene, and linear low-density polyethylene; polypropylene compounds such as polypropylene; polyurethane compounds such as polyurethane; and polyalkylene p-oxybenzoate compounds such as benzoate. The compound is directly formed into a resin sheet or net as the base sheet, or is formed into chemical fibers from which a sheet, net, nonwoven fabric, or cloth (textile, knitted goods) is prepared as the base sheet. Such sheet, net, nonwoven fabric, and cloth (textile, knitted goods) may also be formed from natural fibers (cotton, linen, silk, and pulp), or from a mixture of the aforementioned chemical fibers and the natural fibers.
For forming the base sheet, however, it is preferred to not use substances inhibiting crosslinking of the silicone compounds coated. It is also preferred to not use resins producing denaturation in the silicone compounds coated. More over, it is preferred that base sheet does not contain additives such as antioxidants, plasticizers, UV absorbers, and pigments which may cause denaturation in the silicone compounds coated.
The base sheet preferably has high wettability for uniform and thin coating with the silicone compounds. In order to improve the wettability, a base sheet formed of polyethylene film or the like can undergo corona discharge treatment prior to coating with the silicone compounds.
In the invention, coating of the base sheet with the first silicone compound, the second silicone compound, or the mixture of the first and the second silicone compounds can be carried out by use of an air doctor coater, a blade coater, a rod coater, a knife coater, a squeeze coater, an impregnating coater, a reverse-roll coater, a transfer roller coater, a gravure coater, a kiss-roll coater, a cast coater, a spray coater, a curtain coater (flood coater), a calender coater, an extrusion coater or the like.
In method (1), in order to obtain excellent effectiveness in preventing the adhesion of stains, the primary coat formed of the first silicone compound preferably has a thickness of at least 0. xcexcm or more (more preferably, a thickness of 0.3 xcexcm or more). The surface coat formed of the second silicone compound preferably has a thickness of at least 0. xcexcm or more (more preferably, a thickness of 0.2 xcexcm or more). In methods (2) and (3), the respective silicone compounds preferably have thicknesses similar to those described above, if the compounds are clearly separated into a substantial double-layer structure.
In the invention, fixation of the first silicone compound or the mixture of the first and the second silicone compounds on the base sheet can be carried out according to crosslinking methods adopted commonly for silicone compounds. Examples of the methods include condensation crosslinking and addition crosslinking by heating, cationic polymerization and radical polymerization by UV, and radical polymerization by EB. However, contamination with crosslinking agents, initiators, terminators, and other additives may result in marring the effectiveness of preventing the adhesion of stains. It is preferred, therefore, that the amounts of such substances added be as small as possible.
The sheet coated with the silicone compounds of the invention can be used for various purposes such as raincoats, bibs for babies and infants, aprons, lunch mats, tablecloths, boots, underwear, back sheets of sanitary goods such as diapers and sanitary napkins. Particularly, the sheet is useful for goods having high possibility that protein-containing stains may adhere.
The invention will be illustrated in more detail with reference to the following examples, but these are not to be construed as limiting the invention.